1) Field of the invention
The present invention relates to a method and apparatus for controlling torque variations in an internal combustion engine by changing an air-fuel ratio in accordance with the magnitude of the torque variations caused by variations in a cycle-to-cycle combustion in an internal combustion engine, or variations in the speed of the internal combustion engine.
2) Description of the Related Art
Generally, a vehicle having an internal combustion engine is driven by the torque generated by the internal combustion engine, and when the torque is intermittently lost or a negative torque is generated by a misfiring of the internal combustion engine, the vehicle running is adversely affected by such a negative acceleration force. Namely, a repetition of such a negative acceleration force causes a longitudinal vibration of the vehicle, i.e., surging, and passengers in the vehicle suffer discomfort. Therefore, attempts have been made to improve the fuel consumption and emissions by preventing an adverse affect produced by, e.g., surging due to torque variations. To prevent surging, the amount of torque variation of the internal combustion engine is detected and parameters for controlling the internal combustion engine, such as an air-fuel ratio or the amount of EGR (Exhaust Gas Recirculation) are fed back as predetermined set values.
For example, Unexamined Patent Publication (Kokai) No. 60-122234 discloses a lean burn system of the internal combustion engine wherein a feedback control is adopted. In this lean burn system, the amount of variation of the combustion pressure is detected as the amount of torque variation of the engine, and an air-fuel ratio is varied by controlling a fuel amount injected to the engine so that the detected amount of the engine torque variation becomes smaller than a fixed value. The air-fuel ratio obtained by providing the controlled fuel amount is called a lean limit.
The controlled fuel amount is continuously renewed as a learning value in each engine load area defined by both an intake air amount per one rotation of the engine and an engine rotational speed.
Nevertheless, when the engine is restarted after stopping, the air-fuel ratio may exceed the lean limit, because the learned fuel amount is no longer appropriate due to changes of circumstances, such as ambient temperature and moisture, and thus a surging of the engine occurs until the learning value is renewed and becomes an appropriate value. This situation sometimes occurs when the engine is stopped with the learning value that had been renewed and memorized under a low humidity condition and is restarted under a high humidity condition.
To solve the above problem, Japanese Patent Application No. 63-306124 discloses a lean burn system for an internal combustion engine which prevents a surging of the engine due to a change of the lean limit of an air-fuel ratio caused by a change of circumstances In this improved lean burn system, the air-fuel ratio is controlled to the rich side by reducing a predetermined value at the beginning of the variation limit control of the torque of the engine, after the start of the engine.
Nevertheless, even in such an improved lean burn system as described above, the following problems still exist:
(a) the fuel consumption and emissions of the engine are worsened at the beginning of the variation limit control of the torque of the engine after the start of the engine, because an excess margin for the limit of the variation of the torque is set after the control of the air-fuel ratio to the rich side, which is carried out regardless of the torque variations in the engine at that time; and PA1 (b) the fuel consumption and emissions of the engine are worsened when the variation limit control of the torque of the engine is started under the condition of a low humidity while a reference air-fuel ratio at the beginning of the variation limit control of the torque of the engine is set to a variation limit control value of the torque under a condition of a high humidity, because an excess margin for the limit of the variation in the torque is similar to that of the above-described case.